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How to convert a pressure height (ADS-B) to a geometric height?
As far as I know, ADS-B broadcast a pressure altitude, so I want convert it to geometric one. How is it possible?
e.g in MSG format: http://adsb-decode-guide.readthedocs.io/en/latest/position.html
which kind of altitude is there?

ADS-B provides both pressure altitude and geometric altitude

In MSG format (after decoding) there is only one kind of altitude, which one?

If you need height above the ground you need to know the ground altitude and subtract it from the altitude of the aircraft. ADS-B doesn't broadcast height.

ok, but my question is about the altitude of the aircraft.. where is the zero point?

@delkov See this page, Barometric altitude uses a TC of 9 to 18, where GNSS (geometric) uses a position report with TC of 20 to 22. You need to know what the type code of the position report is that you are decoding.

Do you know which altitude in MSG Kinetic SBS-3 Format?!
It looks like
MSG,1, 0,  0,4006B3,      0,2009/06/19,06:15:17.421,2009/06/19,06:20:17.421,BAW4J,17750,381,097,51.4854,-1.9028,-2496,4244,0,0,0,0

ADS-B gives both pressure altitude and geometric altitude.
In the position messages you will find the pressure  altitude which is essentially a pressure converted to an altitude. For this conversion the ICAO Standard Atmosphere is used.
In the velocity message you will find the difference between the geometric altitude and the pressure altitude. Since the difference between them is fairly limited a few bits could be saved by encoding the difference instead of the geometric altitude.
If you don't have the geometric altitude and you want to obtain it from pressure altitude (regardless where it cones from) you will have to know the relation between pressure and geometric altitude in the atmosphere.
You can model it according to the hydrostatic equations of the ICAO Standard Atmosphere and correct for actual temperature, lapse rate and local pressure. This gives fairly good results.
For higher altitudes data from meteorological models can be of help to improve accuracy.

Do you know which altitude used in MSG Kinetic SBS-3 Format?! It looks like MSG,1, 0, 0,4006B3, 0,2009/06/19,06:15:17.421,2009/06/19,06:20:17.421,BAW4J,17750,381,097,51.4854,-1.9028,-2496,4244,0,0,0

As far as I know the SBS3 only gives you pressure altitude

This doesn't really make sense because you're asking for something with no baseline reference. What are you wanting a "geometric altitude" with respect to? Mt. Everest? Death Valley? Sea Level? A standard radio altimeter will give you "geometric altitude" from your position in the air to the ground immediately below you; up to 2500 ft.
That is, your barometric pressure altitude could be 30,029 feet. If you're flying over the top of Mt. Everest, your "geometric altitude," as sensed by the radio altimeter, would be approx. 1000 ft. If you're flying greater than 2500 ft. above Mt. Everest, you're radio altimeter won't have a reading, and you'll be reading a barometric height of approx. 32,500 ft.
There is no way to derive your "geometric altitude" from a barometric reading because the earth isn't perfectly flat.

I need the altitude above the ground. please look upd*

Got it. Mins is correct then. I was a little confused as to the wording of your question.

"Geometric altitude" means height above the datum geoid, not above terrain. Usually the datum is WGS84, which I think is standard.

I believe the Earth isn't flat at all. A globe earther, I rather think the Earth is an oblate spheroid, and if you removed the seas, a geoid. Stevens and Anderson from Explorer II were the first people to document a curved horizon when they reached 72,000 ft in 1935. Earlier flights such as the just slightly lower gone Osoaviakhim-1 must have discerned a curvature too, however. Auguste Piccard described a "flat disc with an upturned edge" but that he could have determined a curvature if he had a ruler. I dunno whether he had a ruler in his 2nd flight. Bob White(X-15) reported seeing a curve.

ADS-B uses standard atmosphere characteristics defined in International Standard Atmosphere (ISA). Decrease of pressure with altitude is part of this standard which assume a temperature of 15°C at sea level. A correction is necessary if the temperature is different. This correction is also standard.
In ISA,  mean sea level pressure is 1013.25 hPa, and at low altitude each 30 ft the pressure decreases by 1 hPa.
You may use an online calculator or a table.

I don't believe this is what is being asked for. The reader wants to know their "geometric altitude," which is not based on barometric pressure, but rather on radio altitude - which is only good for about 2500 ft. above the ground.

please look upd*

Fair enough. The wording of the question had me thrown a little bit. I'm pretty sure this is the way GPS altitude is measured, is it not?

Are you sure, that ADS-B broadcast exactly GPS, not pressure altitude?!

@Frank: Altiude in GPS is not above ground (topographic surface), nor even above the approximation of mean sea level, which is named a geoid (an equipotential of gravity, initially the EGM84, and today the EGM96 -- this height is named orthometric height), but actually above the WGS84 ellipsoid (ellipsoidal height). Altitude and local vertical are complex matters.

http://adsb-decode-guide.readthedocs.io/en/latest/position.html which kind of altitude is there?

It's pressure altitude. For purpose of separation it does not matter what kind of altitude is used, but everybody has to use the same definition and pressure altitude is easiest to measure, so that is the standard.
Geometric altitude (note: “altitude” always means above sea level in aviation) is only used in EGPWS and as far as I can tell, never displayed to pilots.
Geometric height (note: “height” means above terrain) is measured with radio altimeter and used also for (E)GPWS, but also displayed to pilots and even read out during approach. It is not, however, transmitted in ADS-B or Mode-S, because it is not needed for the controllers or for collision avoidance.

As far as I know, ADS-B broadcast a pressure altitude, so I want convert it to geometric one. How is it possible?

Only to certain accuracy. If you know pressure and temperature in the area where the plane is flying, you can correct some difference, but the lapse rate also depends on humidity and you won't know the complete profile, so there will be some unknown error that will increase with altitude.


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private jet based of an sr 71
could a private jet be based of an SR 71 blackbird. By this I mean similar styling and shape. Would this be a good basis to make a private jet super sonic (disregarding sonic boom). What would the major adjustments be for the SR 71 to be a private jet?
By this I mean: to make a cabin of decent size; take off and land at non military airports; standalone (not needing refuelling etc) and yet supersonic travel.

A private jet to do what supersonically? If you want something that can carry cameras higher and faster than your enemy's air defence systems can intercept, the SR-71 could be a reasonable place to start. If you want something to get your CEO to their next meeting on time, it'd be a pretty terrible place to start.

@DavidRicherby I have specified in the question.

It would be better a lot cheaper to buy a demilitarized fighter and turn that into a business jet, the SR71 was designed very mission specific, not even close to this kind of use.

Yes, a private jet could be based on the design of the SR-71, but there would have to be a market for it.
Considering how expensive it would be to produce, operate, and maintain, no one would want to buy it, so it will never happen.

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text:
Why do I get a severe headache just before landing?
I had severe painful headaches two times in my entire lifetime and both times I was in flight. I see some common factors in this two occurrences which I thought could be some known pattern so I would like to know if we can take precautions to avoid a headache.
The first time it happened to me in June 2007 while I was traveling from San Francisco to Newark. I clearly remember that I was in deep sleep and the crew member woke me up and asked me to open the blind as we were going to land soon. After opening the blind I was exposed to the sunlight and immediately I got a severe headache. It got settled just before the flight landed. I initially thought it could be due to tiredness, insufficient sleep and sudden exposure to sunlight from dark until a similar thing happened to me after 11 years.
The second time it happened to me was during my return flight last week (Jun 2018) from US to India; it was a connecting flight from Phoenix to London. Just a few (10-15) minutes before landing, the pilot started his announcement about the landing; I got this headache for a few minutes and it settled after a few minutes.
Trust me, when it happened the first time it was such a pain I never felt before in my lifetime and I got so scared that something bad is going to happen to me. The second time it was only a little better but very similar. Though I had traveled more than a couple of dozen times in between these two events, I experienced this only twice and I remember common factors in this two events

Traveling from East-West.
Moving to different timezone and at the destination it was dawning.
Pilot announcement.
Window seat.

Are there any patterns to it and can we take precautions?

Your sinus passages were obstructed and it was atmospheric pressure squeezing your skull as the increasing pressure on the descent tried to equalize the pressure.
The air in your sinuses escapes much easier than it goes back in so the problem is always after a descent.
Feels like someone trying to drive a nail into your skull between your eyes.   Plug your nose with your fingers with your mouth closed and blow GENTLY and you will feel the air enter your sinuses and relieve the pressure.
Good idea to take a sinus medication like Otrivin with your travel kit if you are having that problem regularly.

Good answer. I would add that there can be more than one nail and they can be distributed across your forehead and upper cheeks.

Yes depending on how many sinus cavities are blocked.  Pressurization systems generally limit altitude changes to not more than 500 fpm, which most people can tolerate, but if they are blocked enough, even that is too much when pressure is increasing.  You can have blocked sinuses without feeling like you have a cold.  If I was going flying and although didn't have a cold I suspected my sinuses were congested, I'd sometimes take a Sudafed (no side effects for me), and I could almost feel with wind flowing around the little galleries in my head, the passages were so open.

This phenomenon is described as a "Sinus Block" here: http://webapp1.dlib.indiana.edu/virtual_disk_library/index.cgi/4300833/FID2415/ATPUBS/aim/p0801002.htm

Strange how many people have experience about nails in the skull ;-)

Just note: if you try to gently blow/push air into your nose while holding your nostrils and you don't feel air rushing back into your sinuses, don't push harder. It's likely you have a blocked sinus from a cold or infection. In this case you'll have to suffer through the 'headache' when landing and wait for your sinuses to equalize normally, or take a sinus medication after landing.

@Robotnik Yes I should have mentioned that.  You can damage your eardrums if you blow hard enough. Thanks.

I experienced "the nail" once, fortunately I was at the controls and there was an engine so I could stop the descent immediately and then climb and then do the Valsalva maneuver before beginning to descend again.  It was amazing how sudden the onset of the pain was. My memory isn't all that clear but I suspect that I attempted Valsalva alone before climbing and it didn't fully fix the problem.

Be sure to make sure that you don't have someone actually driving nails into your skull...

All in all, sounds like an equalization issue. Descending in an aircraft the whole way is about equivalent to diving 10 feet. That's not a lot, but it's enough that you'd equalize once or twice.
When you feel the characteristic pressure difference (a dull feeling in your head), equalize, either with a nose pinch or with Valsalva (similar to swallowing with your mouth closed).

One's head is packed full of nerves, all running close to each other and to parts of the head, face and neck that deal with, and are affected by, changes in pressure (as noted in other answers).
Nervous pathways are involved in the phenomenon of referred pain, in which the site of pain-causing activity is different from but related to the site at which pain is experienced.
I don't know enough however to know whether the sinuses or ears can cause a headache in this way - normally, pressure changes cause splitting pains localised in the front of the head or the ears, rather than what would normally be described as a headache, which is usually a more generalised pain than "a pain in the head".
I once, as a teenager, had the experience of descending through indescribably beautiful cities and canyons of sunlit clouds on a summer evening over Rome. I was transfixed with emotion at the heavenly magnificence that was unfolding itself through my window. The next thing I knew, without any warning or preamble, was the sensation of someone inserting a hot soldering iron into the root of one my upper teeth.
The pain was excruciating, I have not experienced anything like it before or since. It endured for hours, but by the following morning had faded.
The teeth are very sensitive to emotion, apparently (according to dentists). But, this could also have been an effect of changing pressure on the nerves in the face and head.

Daniele I once had a toothache that I thought was due to a cavity but the dentist took an X-ray and said it was due to sinus issues due to hay fever.  So your tooth issue was almost certainly due to sinus issues relating to descending as described in other answers.

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What do I need to know to design contra rotating propellers for a powered paraglider?
I wonder if anyone has any technical information that would assist in using the correct Size & Pitch Contra rotating Propellers (CRP) on this Foot launched Powered Paraglider engine ?
We are using CRP to Eliminate the dangers of Torque Twist associated when using a single propeller.
I am looking to use standard 100~125cm (40~49") Propellers that have approx. 28~22 pitch.  Noting the Forward prop would need to be a Puzzle type prop, or we would have to increase the forward Hub size, so that it would slip over the Aft prop hub, thus adding more weight & complexities.
This pre-production engine maximum operating speed is planed not to exceed  3'500rpm, so if the Prop output shafts are geared 1:1 with the piston speed or less, the props will also be rotating at 3'500rpm or less.
This is the current CAD design work, the 4stroke, 150cc engine, current weigh approx. 12kg, so it is a very small engine.
So, I hope someone can understand the unique workings of this engine design and move past that and recommend best size & pitch of after-market propellers that would Fit these hubs, preferably also with bolt pattern measurements ?

What kind of techical information are you after?

Out of curosity, why so much complexity for a paraglider?

Why are you writing comments on your own question? Delete them and improve the question so that all the info required is in the one place. Improve the question!

The link has several pictures about an engine and drive mechanism, couldn't see the propellers. Can you add the specific mechanism in your mind, if it's a review question. Otherwise several aspects of design must be included in answers, making it too broad to answer.

Using two counter-rotating props does indeed eliminate torque issues. The co-axial configuration has been around for a while, like the successful Breguet Dorand of 1936. The thrust, power and propeller disk issues are a bit complicated, let's start with a qualitative discussion.

Two 2-blade co-axial rotors compared with a single 4-blade rotor of the same size.
To generate a given thrust, two counter-rotating 2-bladed propellers use less power than a single 4-bladed propeller of the same size - if they are mounted far enough away from each other. Separating the two propellers helps in reducing power: the closer together they are, the less the power advantage. Separate them 20% of the rotor radius, and you still have a 7% power advantage over a single 4-blade propeller, as computed by Kamov (reference). Further away results in less power required. Note that the 7% power advantage may not account for losses from the dual gearing trains required.

Co-axial 2-bladed propeller compared with two 2-bladed separate propellers of the same size.
Momentum theory predicts a power penalty of $\sqrt{2}$ = 41% when two rotors are placed co-axially without separation, compared with the two identical rotors being operated in isolation. Test results have indicated that that is an overestimation: 20% is more realistic.

We're limited to commercially available sizes and pitches and are using fixed pitch. That means that we cannot optimise for the exact circumstances. My recommendations for the design process would be as follows:

From your link I see that there are 40" and 49" blades available. Go for the 40" props.
Your link gives an output RPM of 3500. With a 40" prop, the tip speed becomes (3500 RPM * $2\pi$ rad * 0.5 m / 60 sec) = 183.3 m/s, about Mach 0.55. Forward velocity is around 20 m/s. The velocity triangle at the tip has an angle of tan$^{-1}$ 20/183 = 6.2°
A typical symmetrical profile such as NACA 0012 has a maximum L/D at angle of attack = 4° at M=0.6 (reference).

Add this to the velocity triangle and you end up with a blade tip angle of 10°. That equates to a blade pitch of 22 inch for the forward propeller. Select the standard pitch closest to 22 inch (24" ?). The next one up to 22" should be the best choice, because the aft propeller will be pulling in air and add to the free stream velocity.
The aft propeller runs the accelerated air from the forward propeller and must have a higher blade pitch than the forward propeller. Select the next available pitch up (28" ?).
Naturally, one propeller must be clockwise and one must be counter-clockwise.
Both propellers should be 2-bladed. That will result in four blades in total, more than enough for this application. Adding more blades will cost horsepower and not result in any useful thrust increase.

For the thrust per propeller, if I compute that from a $C_L$ of 0.55 and integrate that over the blade radius from 5 cm to 48 cm (to account for tip losses), I get:
$$ T = C_L \cdot \rho \cdot A \cdot \int_{0.05}^{0.48} \omega \cdot r \cdot dr$$
$$ = C_L \cdot \rho \cdot A \cdot \frac{\omega^2}{2} \cdot (0.48^2 - 0.05^2) $$
$$ = 0.55 \cdot 1.225 \cdot 0.05 \cdot \frac{366^2}{2} \cdot (0.2304 - 0.0025)$$
With

$C_L$ = 0.55
$\rho$ = 1.225 kg/m$^3$ at sea level
A = blade area is about 0.05 m$^2$
$\omega$ = 3500*2$\pi$/60 = 366 rad/sec

= 514 N per propeller. Note that there are assumptions on propeller profile and blade area in there. $\alpha$ may not be exactly 4° so lets take a 10% lower value of 450N per propeller = 900 N total.
Power required to generate this thrust is the blade profile power plus the induced power from thrust generation. When I look up the $C_T$ and $C_P$ factors of the propeller disks, I get a $C_P$ of 0.001, resulting in a power P of
$$ P = C_P \cdot \rho \cdot A_{disk} \cdot {V_{tip}}^3 = 0.001 \cdot 1.225 \cdot 0.785 \cdot 183^3 = 5.9 kW $$
So for two propellers with a penalty of 20%, that would be 14.4 kW = about 20 hp. There are some estimates in here and all depends on the pitch size and if my assumptions of blade profile and blade area are correct, but it seems to be in the ballpark. The product would need to be tested for actual thrust and power consumption of course.
If your counter-rotating setup and gearing works, the main selling points would be absence of torque and a low tip speed resulting in low noise.

Update
Some of the comments are on using larger propellers and one 3-bladed and a 2-bladed, and there is a question on using a spreadsheet for single propellers.

The single propeller spreadsheet may be a compilation of experience on using one single propeller. However, you will be driving two propellers.
A main selling point for your set-up is elimination of torque. Use one 3-bladed and one 2-bladed propeller, and the difference in drag will re-introduce torque. You have 4 blades in total already, no need to add a 5th or more for any purpose.
A larger propeller uses more horsepower than a smaller propeller, and you're driving two propellers. If your engine can drive 2 larger diameter propellers, by all means go ahead.
Your vehicle travels at a maximum of 70 km/h, the propellers won't have an issue with attracting enough air.
Increasing output RPM would be a good way to increase thrust!

@TheRaptor Have updated the answer, about the recommendation on number of blades.

OK, Koyovis thanks, that answers my Question on the odd bladed props. I will now chase up Prop manufacturers so see who will provide there bolt pattern measurements.

@Koyovis In the integral, shouldn't you take the angular velocity as an independent constant, leaving it outside of the integrand...?

@xxavier did a couple of checks, dimenionally the $\omega$ is squared for sure. It is V that is integrated over the blade radius.

@Koyovis  Yes, but the differential expression to be integrated is dimensionally wrong, as the omega isn't squared there... I don't know how did you derive it, but in that differential expression (if the final result is to be right) the omega had to be squared ad treated as a constant, integrating only r dr... Pls check it...

My initial concern is using 100cm (40") props may not be suitable, as the bulkiness of a person sitting in-front of the engine & Cage, may not be aerodynamic enough to utilise the air as it passes around the Pilot & engine ?
That information provided appears great mathematical formula, I will pass on to our mechanical engineer.
I have been using this old Prop-Speed spreadsheet, but I am not sure if the calculations are correct, as I am unsure what the "256000" in the "L24" cell  relates to (=J24*256000/K24) ?

There will be air in front of the propeller, don't worry.

Your link to the spreadsheet does not work.

I just cleared my Cache, it works for me.
It will ask you to download or open a PropSpeed.xlsx spreadsheet

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Does object mass have a significant bearing on wind interference?
I'm learning to fly RC quadcopters, and before I go outside with an expensive, possibly dangerous (if it were out of control and hit something) large unit, I've decided to practice flying for a while on a smaller entry level "palm" drone (if I lose/break it, I won't be out more than $30).
While trying out the palm drone outside, I've noticed that a very tiny gust of wind will knock this thing out of the sky, or at least into a very different direction then intended. Intuitively I want to assume the larger drone will do better against fair-weather Kansas winds; however I've also learned not to assume that my ground based experience translates well into aviation experience. So
TL;DR
Will a larger, heavier drone be less impacted by wind gusts than a smaller palm drone?

To the exact question "Will a larger, heavier drone be less impacted by wind gusts than a smaller palm drone?", the answer is yes. That's a matter of inertia (resistance to motion), which is linked to mass. But this simple answer may not help a lot for your choice. See rather this: Stable flying and hovering drone

In general yes, however if the larger drone has a larger surface area incident to the wind, then it may be just as, or more affected by the wind. The tiny drones are almost impossible to fly outdoors, the larger ones are meant to be outside but usually in very light wind.

I've flown my 250mm size quadcopter in quite high, gusty winds without a problem. The flight controller should keep things pretty steady, but the quad will be "tipped into the wind" to counteract it.

@RonBeyer: The key thing is the surface-to-mass ratio of the drone, which (for drones of a given shape) is more or less a function of its overall density.

Scaling laws are your friend here.
While the area which can be attacked by a gust grows with the square of the length increase, the mass of the drone will grow with something between the square and the cube of length. This translates into higher disc loading of the props, which in turn requires them to operate at a higher dynamic pressure. The same wind gust will change forces relatively less on a bigger drone. It is the same with aircraft: Light indoor models have the same problems outside that you experienced with the small drone.
It gets even better when you look at the capability to resist rotation: Here the moment of inertia is the deciding factor which grows with the product of mass and length squared. This means the bigger drone will better resist attitude changes by the length increase to the fifth power. To be fair, gust moments will increase with the cube of length increase (area times lever arm), leaving you still with an advantage of length squared. This also means that all changes will happen more slowly, which improves the chances that the control system (the MEMS sensors onboard as well as your control inputs) will keep the situation under control.

I would heavily look into the forces of flight: Lift, which counteracts weight.  Thrust which counteracts drag.  I would also look into what software and guidance system you are flying on the small vs the big drone.
I have been a drone pilot for the military and a an owner of a drone company for years.  I can confidently state that the smaller palm drones typically don't have a way to adjust for winds and GPS positioning(other than manual pilotage), however a drone like a phantom for example, can....and they are really good at it I might add.


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Is it worth it? Spending loads of money and years?
I'm a high school student. Over here in India, there are too many people and less jobs. I have a passion and a dream to become a pilot in one of the distinguished commercial airlines of the world. I have researched all the data and know all the facts.
The question which comes to my mind is:
Is it worth spending more than US$80000 for all the training, type ratings and education and still not being capable to end up with a job?
The number of hours required for ATPL are 1500 here but we land up with approximately 310 after University and CPL. How do I increase my flight hours after that? How many years before I'll get a proper job? Is it worth spending so much money and time?
Please let me know guys! I'm not sure whether i should continue in this field or not.

Hi. Welcome to Aviation.SE.  Forgive me for being honest and direct.  if you are asking that question, then the answer is "no".

It is my dream to fly. I'm not sure that i want to give up. But i respect your decision. I'll love to have some other people respond to this too. There's nothing else which i could do besides pilot or YouTube, and my family would probably not support me in YouTube.

There are three distinct questions here. "Is it worth it? Should I continue?" is something that only you can answer: it's purely a matter of personal opinion.  How to get the extra 1200 flight hours is a concrete, answerable question (actually, I'd be surprised if it's not already been answered here). Ditto, how long on average before new pilots get a "proper job".

By the way, why the "helicopter" tag? Major commercial airlines don't fly helicopters.

Rationally, no, it's not worth it.  I was a professional pilot but switched to an office job.  A "real job" will likely pay you more, allow for a better home life, and make for a better quality of life by all standard measures.  Sure, you can justify an aviation career by saying you "love flying" and the low pay and bad hours are worth it.  I love flying, but I discovered what that really means: I love hands-on flying in my own plane on my own schedule.  That's very different from an airline career!  If you fly for a living, what you love just becomes a job. Take care not to kill your passion.

What is your motivation for flying? Is it just because you like aircraft or being in the air? If so you might want to consider ground handling, maintenance, or cabin crew. If what motivates you is having enormous responsibility, methodical work, and performing under scrutiny and pressure, then maybe pilot training will be worth it. But only you can answer that.

I'm not a pilot myself, but can provide some links here:

Burdens of working as an airline pilot
Do pilots make a lot of money?
Why is being a commercial pilot stressful?

You may also ask on PPRuNe discussion board, which is more suited for this kind of questions that are a lot based on individual opinions.
Here is an interesting article from Vanity Fair, that I often cite, about the increasing automation of aircraft, and the related impact on crew members: The Human Factor. The article starts with "Airline pilots were once the heroes of the skies. Today, in the quest for safety, airplanes are meant to largely fly themselves", which gives the tone.

in my opinion if you are passionate for aviation and you are willing to give up your time i suggest you go for it. Because you do a job if you want to do it not just for the money so in my opinion do it if you want to do it for the experience and not for the money!!
P.S i'm just a 13 year old

That's nice. It's about my passion after all, and the job is well paying after a while i guess. It probably gets to around 11,000 American Dollars a month at the highest post. The money is not what i am worried about.

@GameSpout If you aren't worried about the money, what is it you are asking?


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Is it safer to have an airplane inspected twice a year?
According to the Nall report and as voretaq7 explained here, around 11% of fatalities in general aviation are due to mechanical failure and maintenance issues.  Although these numbers are relatively small (about 25 per year) they are a danger that even a very cautious pilot would seem to have little control over.
So, if annual inspections were performed twice a year, does the data suggest  it would impact these numbers significantly?  Please include references to studies and their implications.

This is hypothetical, as we cannot predict if it was safer to have more inspections. Bear in mind though, that the annual inspection is only one of many. GA aircraft in Germany also have 25hrs, 50hrs and 100hrs inspections, which are performed on the engine and sometimes airframe.

FWIW, there are some differing opinions around this. At least one well-known GA guy believes that less maintenance would be preferable to more.

I'm pretty sure there is no regulation preventing a pilot from paying to have the annual inspection performed every six months? Consequently, if in your opinion doing so would reduce your risk, nothing should prevent you from taking such preventative steps, regardless of the regulations, as long as you meet the requirement of performing annual inspections yearly in accordance with the regulations.

@mins, I believe the Nall Report includes only accidents in the United States.  And while 25 accidents a year is pretty small, it's about 13% of the fatal accidents.  What I was wondering, is what can a GA pilot do, if anything, to mitigate this risk?  And unfortunately Pondlife's link demonstrates that increasing the frequency of maintenance is not likely the solution.

This question is the subject of a [meta] discussion: https://aviation.meta.stackexchange.com/q/3316/753


text:
What material is used to make luminescent placard on civil aircraft?
I'm an engineer working in the civil aviation industry. Currently my company is preparing to make some placards. I've downloaded the drawings from Airbus. A drawing calls up the material of AD400 TV25/3881/83. I've seen the original OEM placard and I remember it's luminescent (it's located next to the emergency door).
I've searched "AD400" and "TV25/3881/83" as the keyword on Google but found nothing useful. I think it might be the spec of the material, but what it really is and where I can get it.
It will be greatly appreciated if someone can tell me more info on this material. It would be much better if someone can tell me where I can buy this material.
Update:
Below is how the placard looks like (a screenshot taken from the drawing):

The diamond 12 calls up the material of AD400 TV25/3881/83. Also the drawing says the base material is " MAKROFOL-E". I know "MAKROFOL" is a kind of PC film and it's a trademark of Bayer, but I didn't find this material on the website of Bayer either.

You appear to be asking what materials you should use to manufacture a piece of safety equipment. You absolutely must not rely on answers from random people on the internet for a safety critical system. Ask Airbus, not us.

I don't know what exactly AD400 TV25/3881/83 is, but if you are talking about a sign like this:

It appears to be plastic and Larchfield sells them. This material AD400 TV25/3881/83 could very well be a type of plastic, but I don't know about it.
In my opinion, you can contact companies which make or sell them (Larchfield or this or this) to get more reliable information.

Hi, Farhan. Thanks for your reply. I just edited my question and uploaded the drawing of that placard. Will this help you recall more info? Thanks.

As you've surmised, it's a type of polycarbonate.  It's also probably impregnated (or reverse-coated) with a phosphorescent compound so that it glows for a while after being exposed to light. (In the old days it would probably have been radioluminescent - phosphorescent compounds doped with radium or similar so they always glow, even when they haven't been exposed to light - but that's generally frowned upon these days.)
You could certainly order the material from Bayer if you need it in bulk (for making your own signs), or order similar materials which are probably cheaper. (Be aware that other materials may not meet flammability or environmental/safety requirements for aerospace applications though: In particular many of these sheets are PVC, which would produce some nasty byproducts in a post-crash fire!)
If you're looking at this for aerospace purposes (for example building a cabin simulator for flight crew training) I'd do as Farhan suggested and contact one of the companies that produces replacement placards and just buy them - it's the ultimate level of realism, and way easier than making them yourself. If you're looking to make placards for your own aircraft out of similar material they might also be able to source it for you (or be willing to do a custom placard run, for a price).


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Will ATC proactively route traffic around bad weather?
When thunderstorms or other severe weather crop up, will the Air Traffic Control in the area proactively route traffic (horizontally or vertically) around the trouble area, or do they leave it up to the individual pilots to request an alternate route?
My intent was asking about commercial aircraft flying IFR, however the information about general aviation and VFR is appreciated

Yes, ATC will reroute flights that are operating on Instrument Flight Rules (IFR).
There are lots of videos of flights getting routed around severe weather floating around on the internet! Atlanta is a great example, as is Memphis, FedEx's hub.
In addition, NASA is working on a product called Dynamic Weather Routes that will help controllers be smarter about amending planned or unplanned weather reroutes.
Sort of, for aircraft operating VFR.
VFR flights that are talking to ATC may get advisories about weather and suggested headings from the controller. I've received notifications about things like 'moderate to heavy precipitation 5-10 miles ahead' many times when operating on VFR flight following.

Those are some cool videos, thanks for linking them.

The system is a combination of both. The controller will have a view of the weather radar for the area, and can route traffic around bad areas. They will at least advise the pilots of the bad weather that they can see, and can offer the pilots a deviation or let them know what previous flights have done.
However, the pilots have a better view with the weather radar on their plane, and the pilots have the ultimate decision as to what they will do. The pilots may see a weather return they don't like and ask for deviation around it. Even if ATC denies the request, the pilots are ultimately responsible for the safety of the flight, and should do what they feel is best, even if that means deviating without permission.
There is an interesting article here that is a bit dated, but should still apply to the general idea of how ATC handles bad weather.

Good answer, except maybe it should be made more clear that not all pilots have a weather radar onboard. Large transport aircraft will generally have them, but light aircraft often don't.

My experience from flying Commercial IFR in Europe is that ATC never proactively re-route around bad weather, but are however usually expedient handling requests from pilots (heading changes to circumnavigate), based on airborne weather radar data.

Pilots do not "request routes" except when entering or exiting congested air space, or Class A air space or IFR conditions. They optionally notify ATC of where they might be going by filing a flight plan. VFR pilots can also request flight following. When flight following is activated ATC will notify you of other aircraft or weather conditions that could cross your path.
In uncontrolled airspace, ATC will notify you of a dangerous condition if you are on a flight plan that would bring you into contact with dangerous weather or have flight following activated. It is up to the pilot to decide what to do with that information. In controlled commercial airspace (Class A), ATC will actively route aircraft around dangerous weather conditions.

I'm not sure I agree with you. When I file an IFR flight plan, I am definitely requesting a route. There's just no promise I will receive that route from the ATC computers. I also can request routes in-flight - GPS direct, different routings, different altitudes - all of these are things I can request from ATC.

@egid I modified my answer to include class A airspace.

I still disagree with you, even with a mention of Class A. The only airspace in which pilots can operate IFR without a clearance is Class G, and I highly doubt people do it often. In any controlled airspace on an IFR clearance, a pilot can request an amended route from ATC. Like I said - they may not have that request approved, but you can request whatever you like.

I think the confusion here is that you're talking about VFR flights only, and skipping the IFR side of things.

@egid My understanding is that a clearance is only required when conditions are IFR. Direct quote from the FAR: "An ATC clearance must be obtained prior to operating within a Class B, Class C, Class D, or Class E surface area when the weather is less than that required for VFR flight."

I can fly VFR, maintaining ≥VFR minimums of a thunderstorm (including beneath one), and be legal but incredibly stupid. I can also be on an IFR clearance and request a route. I'm not sure what you're claiming here.

@egid I am not claiming anything. I am telling the OP that pilots do not "request routes" unless they are flying in special conditions like Class A or IFR in IFR conditions. I almost never request anything from ATC except flight following sometimes.

If you're operating IFR, you're IFR. There technically isn't even such a thing as "IFR conditions", just IMC. Your "unless" clause is causing your answer to be very incomplete, which is why I'm pushing so hard on this.

@egid No problem, I don't have an instrument rating so I defer to you and welcome the help. However, my understanding is that if did have a rating and I wanted to fly, say, from Sterling to Laconia and I thought I might run into weather, I could file IFR, but still fly 1200 all the way as long as I encountered no weather. I.e., I don't need to contact Boston Center and get a clearance until I hit weather and activate the IFR plan. My only restriction as far as I know is that if I fly for more than 2 hours I need to call ATC to extend the plan if I am not activating it, so it does not cancel.

At that point, why file? You can get a 'pop up' IFR clearance at any time. That said, why not just fly IFR? You can even go VFR-on-top at that point and you have the best of both worlds. As a CFII I just don't get why, if you think you'd need to go IFR, you wouldn't file from the beginning. Also, as IFR flight plans are from a point A to point B, there's a good chance that you'd have a harder time getting your clearance once you're halfway there.

This answer would be generally correct if "pilots" were changed to "VFR pilots." And, of course, you can't fly VFR in Class A anyway, so that would also eliminate the need to mention Class A. Also, egid is technically right about "IMC" being the correct term rather than "IFR conditions," though the latter is commonly used, too, even in official National Weather Service forecast products.


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How stable is the "customer" power on a commercial aircraft?
This question on Travel.SE got me thinking... Just how stable is the power that's available via the outlets in the passenger cabin on a modern, commercial aircraft?
My assumption in my answer that it's probably quite clean was based on these things, but I'm not sure if or how accurate these assumptions are:

Power for avionics needs to be clean to ensure there are no spikes or dips that would cause avionic failures as this could lead to otherwise avoidable emergency declarations.
Since the plane is already providing clean power to the avionics, the power coming out of the transformers to step it down to 110V, 60Hz for the outlets should also be pretty clean.

Are these accurate assumptions, or is the effort to provide nice, super clean power to the cabin outlets too much work/too expensive to bother with getting it any nicer than the sketchy quality stuff the power company provides to your house?

Are you only looking for answers with 110V, 60Hz or are you also looking for USB power?

Valid question on the USB power. I would assume (with all inherent dangers) that the step-down to USB would be done at the same relative quality as the step-down to the standard line/mains power levels. Therefore USB would be included somewhat by default.

Avionics components are likely using 28 V DC which can be rectified to the level of defect required, while 220V or 110 AC is obtained by a static converter. There may be not relation between AC and DC default level. What do you mean by "stable"? Voltage or phase/frequency or harmonics? That said, today's mainstream equipment doesn't require a good filtered AC source like in the past. Often they can accommodate between 220 and 110 V without a switch.

@mins - stable as in "I'm a layman and I don't want my laptop/charger blowing up when I plug it in." i.e. no need to worry about providing a surge suppressor and/or UPS when I fly.

Clean power from the power company? Have you tried putting a scope on your home outlet? Especially if you live in an industrial area... The voltage on your home outlet can swing from 110V to over 120V. If you live near a factory, the frequency can swing quite a bit too, usually 58-62hz is considered "good" power, but large factories (like smelting) starting up a piece of equipment can drag the grid frequency down as low as 55hz.

@RonBeyer - There's a reason that every PC & all my home theater equipment is behind a UPS. My last statement was intended to indicate "...any nicer than the crappy power that the power company provides...". I'll reword it a bit to make my intent more clear

Most switch-mode power supplies (such as used for laptops and other chargers) are very tolerant of frequency changes (the first step in sonverting to DC) and  voltage changes (these days many are rated for 90-264V).  Spikes and brownouts are a different matter

It's absolutely terrible.  I have never once flown on an airplane where I was able to plug in my laptop and get power.  The sticker on the power brick says 180 W, which really isn't all that much these days, but every single airliner I've ever been on that (supposedly) has power has failed to supply it.

Interesting, @MasonWheeler. On my most recent trip (my first in several years) I was able to keep my phone & tablet topped up via both USB & wall-wort power. YMMV.

One thing that hasn't been mentioned in the answers yet is that typically (in every jet that I have flown anyway), the passenger outlets have their own inverter(s) and aren't related to the AC power for the aircraft/avionics (unless they are all powered from the same DC source to begin with). This is how they end up with the 60HZ power, you don't just "change" the frequency of AC power like you can the voltage by using transformers.

There is a pretty great article on the design of the 777 electrical system here.

Power for avionics needs to be clean to ensure there are no spikes or
  dips that would cause avionic failures as this could lead to otherwise
  avoidable emergency declarations.

The avionics and passenger power systems are two completely different systems. The 28V instrument power system is pulled right from the source,

Flight instruments on the 777 are primarily powered by 28 Volts DC.
  Direct current power is supplied by three-phase transformer-rectifier
  units. There are four such units each rated at 120 Amperes.

This pulls right from the generator/alternator output of the plane. Everything ranging from your classic Cessna 152 all the way up through a 777 regulates their alternators/generators with voltage regulators and fuses to protect against current overloads. These things work in tandem to protect the avionics and internal power systems of the plane. The 28VDC system for the avionics is also connected to the battery system which prevents dips should the generators have issues outputting enough power or quit all together.
The rest of the plane is powered by 110/200V 400Hz system.

The primary electrical system is a 3-phase, 4-wire, constant frequency
  400 Hz, 115/200 Volt system, which has been the industry standard on
  commercial jet transports since the Boeing 707

Interestingly it's 400Hz, I'm still looking to see how they get that to 60Hz.

Since the plane is already providing clean power to the avionics, the
  power coming out of the transformers to step it down to 110V, 60Hz for
  the outlets should also be pretty clean.

The power is quite "clean" but you have some incorrect assumptions here. The power is not really stepped down. That is something that occurs on the power grid since most power lines on the street carry a significantly higher voltage than what you have in your house plugs. In this case we have the luxury of choosing an alternator/generator system that is close to (most likely slightly above) our required voltage. Slide 4 from this presentation from Boeing alludes to the fact that the system outputs 115V (and is presumably conditioned) and is then split to the cabin as well as the 28V DC bus. Power does not come out of the transformers it comes out of the generator/alternators it then is conditioned and transformed as needed. Stability is achieved though numerous conditioners and regulators, see the article I linked for more details on them.
Don't worry, it's safe to charge your laptop on a jet.

So my assumption of clean AC/USB power for charging was correct, it was just my thoughts on how it gets there was wrong.

Generally yeah. Most power consumer power supplies (chargers) are capable of outputting pretty clean power from a wide range of inputs. You may want to use a small USB charger on the 120V socket in a plane to clean it up yourself if you are worried but I would not worry about aircraft power generally speaking.

There clearly must be a rectifier/inverter circuit involved somewhere. I would think that the characteristics of the output power would be almost entirely dependent on the characteristics of that rectifier/inverter circuit.

To underscore something that's implied here: a transformer adjusts AC voltage; it does not affect frequency. So changing that 400 Hz. power to 60 Hz. requires something other than a transformer.

They likely don't get to 60Hz or 50Hz for that matter. 400Hz is actually a fairly standard industrial AC frequency - the higher frequency allows for smaller transformer components. Switch mode PSUs used for laptops, phones, etc. rectify to DC before regulation which is why they can cope with 110V/240V and 50Hz/60Hz. They should as a result also cope relatively well at 400Hz.

AC to AC conversion with frequency change from 400 Hz to 50/60 Hz is obtained by transforming AC to DC first (usually to 28 VDC to be compatible with batteries) and then using a static power inverter to convert from DC to AC. The inverter performs the voltage transformation in the same step.
From an EASA certification perspective, electricity quality is measured at the output of static power inverters.
The requirements are described in ETSO-C73, with an exception about the frequency, as for passenger outlets the frequency must be 50 Hz or 60 Hz, depending on countries, instead of 400 Hz. This exception is described in this document:

Deviate  from ETSO-C73 attached FAA Standard for Power Inverters
  paragraph 2.3  and provide 50 Hz +/-1 % or 60 Hz +/-1 % instead of the
  required 400 Hz.

ETSO-C73 itself refers to FAA requirements:

2.3 Frequency: The frequency of the inverter under all conditions of
  load and test environment shall be 400 cycles per second ±1 percent
  at the input voltages specified in 2.2a. and 2.2b.
2.4 Voltage Output: The average phase output voltage, under the
  conditions of input specified in 2.2a. and 2.2b. and under a ll
  conditions of test environment, shall be 115 volts a.c. +5 percent -7
  percent
2.5 Waveform: The output waveform shall be substantially sinusoidal
  and contain less than 7 percent harmonic distortion under
  all load conditions not exceeding 110 percent rated output.

What does that mean?

Voltage value tolerance +5%/-7%. This is pretty nice and equivalent to what we have at home in industrialized countries. I addition, common devices are very tolerant to voltage variation, as they are simple to eliminate using a high value capacitance.
Frequency tolerance ±1% means ±0.5 Hz. Most, if not all, devices don't rely on a very stable grid frequency. It was the case of first digital clocks to save the price of a crystal oscillator, but now crystals and PLL are used everywhere when a stable clock is required.
Waveform with a maximum of 7% harmonics. This is the usually the most important tolerance. It describes how much the voltage wave can differ from a pure sine wave.

Any periodic wave of frequency f which is not exactly sinusoidal can be broken down into a set of pure sine waves which frequencies will be f, 2f, 3f, 4f, etc, and inter-products. Breakdown is obtained by a Fourier analysis. The multiples of f are named harmonic frequencies, and in practical this is the same as if there was multiple mono-frequency generators.

Some of those frequencies can interfere with receivers or disrupt other circuits due to impedance changing with frequency. In this case the amplitude of the harmonics must be weaker than 7% of the fundamental wave.

For the power grid where I live:

Voltage tolerance is ±10%
Frequency tolerance is ±1%.
For harmonics, this is more complex as the customer's load affects harmonics, there are mutual commitments which are respectively for 2nd, 3rd, 4th, 5th, 7th orders: 2%, 5%, 1%, 6%, 0.5%, 5%.

USB sockets voltage is defined in the USB standard:

5V +0.25V/−0.55V (+5%/-10%) for USB 3.

There is no difficulty to obtain it from any DC source to minimize conversion stages, losses and heat. This can be done by a push–pull converter. USB power is likely obtained from 28 VDC.

Missing a word in This is a very generator, (...) ?

The difference from a pure sinusoidal waveform is most likely going to be in the form of a step-wise approximation.  This thread points out that desktop power supplies with active PFC (power-factor correction) can have trouble with UPSes that approximate a sine too poorly, and that having the voltage sit at zero for some time is a problem.  Presumably 7% THD is small enough that no power supplies have a problem, and modern laptop PSUs are surely designed to handle airplane power.

@PeterCordes This problem seems specific to having two inverters in series (the UPS and the internal power supply) and can have two reasons: The UPS delivers current which returns to zero more than twice per period (poor design), or the internal power supply loads the UPS in non-linear way which is often the case of a pulse width modulation power supply, this can create resonance in the load under certain conditions.

Surely it's not just inverter -> rectifier -> inverter that's causing the problem described in that thread.  People were saying it only happens with active PFC, and that older PSU with passive PFC or no PFC were fine.  (Ironically, older PSUs like that would do best with a pure square wave input.  IDK what active-PFC supplies would do with a square-wave input, since I haven't totally grokked active PFC).

So... what are the chances of getting a portable oscilloscope or data-capture card through security in your carry-on to investigate this?  :P  I bet they love it when people show up wanting to plug home-brewed electronics into airplanes.

Somehow, and answer & comment thread came through the wormhole from [Electronics.SE]!

50 Hz +/- 1% = (50 * 0.99) Hz to (50 * 1.01) Hz = 49.5 - 50.5 Hz. Not sure where you got your +/- 2 Hz from. At 400 Hz it'd be 396 to 404 Hz, for a span of +/- 4 Hz or a total span of 8 Hz.

I am the engineer who designed the AC outlets for Astronics and True-Blue Power which have been installed on most airliners during the last 20 years. The power quality exceeds that of household power which you get at your wall socket. The 115VAC is within +/-2%. The harmonic distortion at 60Hz is very low with the power factor (roughly 1-HD) was certified as better than 99%.

I probably know you...  ;)

Hi Michael, Darrell here. I had left Astronics in 2006 before you got there. Jeff told me he basically took over all the Empower new designs after that time.


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Why would one plane in this picture not have gear down yet?
This image (as posted in another question) is a YouTube video's thumbnail:

I noticed that the lead twin (737?) has gear down, the 747 behind does not (appear to) have gear down, but the third plane in line (another twin) does have gear down as well.
Why would the 747 not be gear down at this point?

FYIW the twin in the front is an Airbus (318-A321)

There is a 75% chance @Terry sees this meaning there is a 98% chance he has the answer and a 0.001% chance it was him flying that 747...

@DeepSpace It looked like the engine nacel's were flattened on the bottom, hence my 737 guess.

Please tell me that photo was either staged, or taken with a really long telephoto lens.

@jamesqf Well, that fifth plane doesn't seem to have its landing lights on, even, so...

@jamesqf A telephoto lens coupled with the difference in size between #1 and #2 would them appear a lot closer than they really are.

Is it even given that the 747 is second in this sequence? Could it be that with the tele-lens-effect that the third plane from top (twin) is actually the 2nd in sequence, flying at a lower altitude?

@Scrontch given the separation required between planes, that third plane would have to be a very small regional to be closer to the lens, yet appear that much smaller than the 747.

@Scrontch I was thinking the EXACT same thing.  Unfortunately the image is both too small (to see detail from the original image) and compressed (yay, artefacts) to make a proper deduction here.

The image does not appear in the video. Do you have a link to the correct video at the specific approximate time?

@VladimirF - I pulled the image from another question (as was indicated, but was edited out). I've included the link to the original question. I've not seen the video, so I have no idea what the contents of the video are.

@ymb1 I see. OK, well... in that case, I guess Vladimir is out of luck. as I have no idea where the actual image came from... I'll leave it attributed to your question, though, as that was my source and what prompted the question. Since you kindly added the YouTube link, I'd think that'll work as "official" sourcing... (I hope)

@Scrontch that small regional would surely not be that close behind the 747, the wake turbulence wouldn't allow it, so I think you're right, 747 must be #3

The ye third 737 saw the camera, and dropped his gear early to make the 747 look bad ;)   Seriously @ymb1 it is SOP for youtubers to fake their thumbnails. Youtube now allows larger posters to select the thumbnail and they often use a ringer, like a C-17 launching off a carrier.

@JeffUK if you'll reread my comment, you'll see that I said it would have to be a very small regional to be in front of the 747 and still look that much smaller. A telephoto lens will compress distances making the planes look much closer to each other than they really are (search on [photography.se]). Also, see the math in Chris H's answer showing approximate distances between the aircraft

Why does the 747 not have its gear down yet?
Based on the image it is hard to say how far these aircraft are from the runway. The 747 could still be be quite far from the runway and thus does not need to have its gear down yet.
The 747-400 FCOM (NP.21.47 Normal Procedures) says:

At glideslope alive, call:

"GEAR DOWN"
"FLAPS 20"

It looks like the 747 in the picture has already intercepted the glideslope, but this is hard to say from this perspective. Also, operators may have different rules for when to lower the gear.
Then why does the third aircraft already have its gear down?
The most probable reason for this is drag. A flight crew may choose to lower the gear earlier than necessary to reduce its speed. ATC could have instructed this aircraft to fly at a lower speed for spacing and the crew then dropped the gear, which adds significant drag. This is pure speculation, of course.

I second these assumptions based on knowing the approach procedures at the airfield in question (EGLL) and some familiarity with SOPs for at least one airline/type combination pictured here.

Having spend a fair bit of time being paid (by NATS) to watch arrivals at Heathrow about 20 years ago that's a seriously long lens.  I used 8-12x binoculars to get an earlier chance to ID types on arrival and you wouldn't get a view like that even with 12x.  Without knowing the weather conditions on the day, but assuming they're typical, you can tell from the haze that the planes are far apart - look how much less sharp each one is than the one in front of it (the 5th is recognisable as an aircraft but only just).  I suggest that the later twins are larger than the one at the front, further shortening the appearance, thus they're all further away than they look.  This would also explain why the last doesn't have its landing lights on.
Adding up the wake vortex separation (relevant tables) as an estimate of the length of the shot and assuming we have a Medium (seperation not dictated by wake vortex so I've used a low figure of 2.5 NM) followed by 4 Heavies (4 NM required for a Heavy following a Heavy) we have 14.5 NM from the first to the last.  With no ground in sight we can assume that the A320ish is still some way from the threshold, so it's likely to be just that it's further away than it looks.
CptReynolds reckons no. 3 is an A320 (like no. 1).  If so, they're minimum* 7.5 NM apart and no. 3 is (by similar triangles) 2.5x as far away as no. 1.  That makes no. 1 4.33 NM from the observer. We lack metadata and any direction or even time references but this map I found   shows plane-spotting locations roughly 300-400 m (or 0.2 NM) E of the visible start of 27L and 27R (I assume the airport is on westerly operations because of the prevailing wind, and I won't consider displaced thresholds).  If the observer was at one of those locations, that would mean we have, with rounding, something like:
A/C no. A/C type  Min distance from runway (NM)
1       A320      4.5
2       B747      7
3       A320      12
4       ?         14.5
5       ?         17

At this point, I admit to knowing much more about photography, maps etc. than aviation operations, and will let the experts weigh in on reasons (my suspicion is that no. 3 is limiting its speed)

* Of course there will be a margin on top of the minimum, but LHR runs pretty tight so I'll ignore the fact everything is probably a little further away.

The question really seems to be not "why doesn't the 2nd aircraft have its gear down" as "why does the 3rd", given that it's still some way out.

BTW similar calculations are made in the comments under the youtube video

That's very interesting information and great insight, but I'm not sure how it answers the question - why does the 2nd plane, which is closer (as you confirmed) not have its gear down whilst the 3rd plane, which is obviously farther away, already have its gear down?

Chris, I think you got carried away with addressing one of the comments to the question, rather than the question itself.

@DavidRicherby possibly.  This was going to be a comment but adding the links meant there was no way I could compress it.  I won't change much, at least for now,  but will add one important point

I‘m pretty certain no. 3 is an A320 family aircraft, even though it might (for the pictured combination) not be the optimal sorting to stack‘em that way round, separation-wise.

@CptReynolds you could well be right.  I might get the chance to look at the photo again on a big screen and draw some conclusions about distance in that case.  It wouldn't change the overall length of the queue very much, as the separation in front of an A320 would increase, and behind would decrease.

@David Richerby: But the answer to the original question is so simple.  Because the crew didn't hit the "gear down" switch yet, no?

@jamesqf That would be a very poor answer, as it doesn't explain why didn't they hit the switch yet, given that the pilot four miles behind them already had.

@CptReynolds I've run the numbers (with one more calculation to follow I think - no, not enough data to work out the lens length etc.)

Larger plane more drag on the wheels perhaps? I'm only guessing, but it would make sense to me, the larger size of the second plane would mean there was already extra drag on it compared to the smaller planes, and lowering the gear too soon might have reduced their air speed by too much.

This seems very speculative. Sure, the larger gear of a larger plane will cause more drag in absolute terms than the smaller gear of a smaller plane, but you don't give any basis for suggesting that the gear of the larger plane would contribute a greater proportion of total drag than on the smaller plane. Sure, it might but it might not, too. We're looking for answers that are more authoritative than "I'm only guessing."

Well, it’s not only the drag but the total sum of force that counts. E.g it‘s also much higher weight pushing it down the glidepath, right?

I was merely putting forwards a possible factor for the planes gear still being up, but as you say David, "it MIGHT but it MIGHT not, too" (way to hedge your bets David) try sticking your hand out of the window of a car doing 60/70 mph. I'm guessing you 'Might or Might not' feel the drag that such a comparatively small thing can cause, now compare that to the wheel size of the plane, I would imagine the drag would be at least 'slightly' greater. I am not an aviator and do not claim to know anything about avionics, but I do know that the drag from the landing gear can slow air speed down.

The point of @DavidRicherby's comment is that at all the SE sites, we're not looking for "perhaps", "I'm only guessing."` or "I would imagine" answers. i.e. we're looking for facts as can best be determined, preferably with references to sources for those facts. If you have guesses, perhaps', or imaginations, the "comments" are the ideal place for those. You don't (yet) have enough rep to comment in general, but continue contributing and you soon will.

@Andy Read my comment more carefully. I acknowledged that lowering the gear causes drag, which is obviously true. But you claimed more than that: you claimed that lowering the gear affects big planes less than small planes. You gave no evidence at all for that claim and it's not at all obvious that it's true. In fact, I'd expect it to be false.

@ David I read your comment fine thanks Dave, but perhaps you should re-read mine instead, at no point did I claim that "lowering the gear affects big planes less". So perhaps instead of writing about what you think is there, you could actually read what was written.

authors: Vladimir F Героям слава


text:
What is the accuracy of distance and position calculation?
I would like to learn about the flight distance calculation and positional accuracy requirement in aviation.
In practice, great circle distance is calculated between two airports but earth is more accurately represented with an ellipsoid and in this context, global positioning is based on WGS84 ellipsoid.
Is it useful to calculate geodesic (shortest path on the ellipsoid) rather than great circle on the sphere for more accurate estimation of the distance?
When thinking that flight routes are 3D, are 3D distances calculated in flight planning? In addition, are these 3D distances calculated with elliposidal cartesian (ECEF) coordinates (X,Y,Z) regarding ellipsoid height (height(MSL)+geoid height (undulation))?.
Does it make sense to calculate the flight distance in "meter" against "km" precision? If it is in nautical mile, what is the precision? e.g. 2394 nm or 2394.xx nm?
My another question is how accurate the 3D position of an aircraft is determined? What is the future perspective in this context together with satellite-based navigation and augmentation systems?

Is this a homework question?

Positional accuracy is tied to the position source, for WAAS GPS, this is around 7.6m worst case and about 1m on average. I don't see any need to calculate flight distance sub meter, great circle is accurate enough unless you are landing delivery drones on somebody's doorstep.

Every distance calculation I've ever seen uses WGS84 or NAD83. When they say "great circle" I'm sure calculations are actually using an ellipsoid model.

It would very much help to know what context you want information about.  Airlines?  The military launching a cruise missile?  Me flying the Cherokee to a dirt strip somewhere?

The context is civil aviation.

The aviation industry is in the process of moving to Performance Based Navigation (PBN) within Required Navigation Performance (RNP) airspace.  Using this structure the primary performance is measured by the Total System Error (TSE) which must be less than 2 times the RNP value 95% of the time. Example: In RNP 1 airspace the aircraft must maintain positional accuracy within 2.0 NM of its desired path 95% of the time. The standard for navigational equipment (FMS) is defined in RTCA document DO-283B, Minimum Operational Performance Standards for Required Navigation Performance for Area Navigation.
The TSE for an aircraft is broken down into three components; path definition error (PDE), position estimation error (PEE), and path steering error (PSE).  Your question directly relates to PDE.  The accuracy of PDE depends on several components, the primary one being the accuracy of the positional (fix) information published by the regulatory agency (FAA in the US) contained within the navigational database. For enroute and terminal airspace, the accuracy of fix information is less than 0.01 minutes of latitude and longitude or about 17 meters.
Using the data from the database and the WGS84 earth model, the FMS will compute lateral path data.  Lateral paths are computed as a geodesic on the surface of the WGS84 ellipsoid.
DO-283 specifies the following data (among other) resolutions for display and entry:
Distance:  0.1 NM for values <10 NM, 1 NM for values >=10 NM
Fix latitude/longitude: 0.01 min
RNP:  0.01 NM for values <1.0 NM, 0.1 NM for values >=1.0, and <10 NM, 1 NM  for values >=10 NM
EPU: 0.01 NM for values <1.0 NM, 0.1 NM for values >=1.0, and <10 NM, 1 NM for values >=10 NM
Present Position latitude/longitude: 0.1 min
DO-283 also states rounding of displayed data is desired.  For that reason internal FMS computations normally are performed to 1 decimal place more than the displayed resolution and then rounded.
From the above you can see that the displayed distance of a leg should be within 0.1 NM for distances less than 10 NM and 1 NM for longer distances.
As for your 3-D question, lateral (LNAV) and vertical (VNAV) navigation are computed separately.  VNAV is still predominately barometric based except for precision (GBAS) and near precision (SBAS) approaches which use WGS84 geometric height.
For positional data, along with the present position, the system must calculate an estimated positional uncertainty (EPU) as shown in the table.  The EPU calculation is quite complex and varies based on the source of the positional data (GPS, IRS, etc.)  This value when added to the PSE (cross-track deviation + errors) should be held within the RNP value.  As you can see form the structure of the EPU requirement, it is the most significant component of the system error.
Summary:
Calculating a geodesic path is required by the MOPS. Great circle is not acceptable.
Paths are calculated using lat/lon and baro altitude coordinates.  Distances are output in nautical miles; not feet, meters, or km.  Internally, the systems can use what ever they want.  Altitude is in feet or meters, depending on airspace rules.  The 3-D path is used to predict time of arrival at each fix along the route.
Positional accuracy (EPU) is calculated based on the selected nav sensor(s) using a number of statistical methods including Kalmann filters.

Really thank you for the detailed information. Could you please explain the distance precision for whole path? e.g. aaaa nm or aaaa.aa nm?

It's important that we separate internal accuracy from display accuracy.  Obviously, the internal accuracy has to be greater than or equal to the display accuracy. Internally, all distances are computed to 0.01 NM.  that's about the best you can do since that's about the accuracy of the fix information (0.01 min of lat or lon ~ 0.01 NM).  You need to use the best accuracy you have as you build the flight plan to minimize the additive effect of the error over the entire flight -- error/leg x #legs = total error.

For a display, it's about providing data useful to the pilot. Flying a 300 mile leg at 450 kts is a 40 minute leg.  That's one mile every 8 sec, so displaying XXX NM is reasonable while cruising.  When you get near a fix the pilot needs to anticipate the upcoming turn, so the distance-to-go display shifts to XX.X NM within 10 NM of the fix.  At cruise, those tenths tick off at less than a sec. A hundredth of a NM display would be unreadable.  On approach, it's still less than 3 sec per .1 NM.  And these are minimums. Some FMS display XX.X below 100 NM and X.XX below 10 NM.

The requirement on Performance Based Navigation (PBN) equipment is that:
(1) The Total System Error (TSE) must be equal to or less than the
   Required Navigation Performance (RNP) value for the procedure for 95%
   of the flight time; and
(2) The probability that the TSE of the aircraft exceeds the specified
   TSE limit (equal to two times the RNP value) without annunciation
   (notification of the pilot) is less than 10^(−5).
My experience (doing planning work and analysis) is that the distance between two airports is calculated 'along the ground' and does not account for altitude.
The distance between two points (e.g., airports) computed using a great circle approximation is less that 0.5% of the distance for ellipsoidal earth (and usually less that 0.3%). Vincenty's algorithm is one good method for calculating distance along the ellipsoid surface. Matlab implementations are available on the web.
The problem with using Cartesian (ECEF) coordinates is you almost have to then calculate a straight-line distance for a path that pierces the earth, rather that a distance along the earth's surface (which is what you want).

I couldn't understand the problem with using ECEF. You have to use ellipsoidal height (Hmsl+Hgeoid) along with other parameters to calculate the 3D position of an aircraft. You're right that straight-line distances will be calculated but in the air not on ground or on a reference surface (sphere or ellipsoid). The distances are calculated on a reference surface not on the ground in fact. Let's assume the difference between ellipsoidal and spherical calculation method is about several nautical miles. Will you ignore this difference?

The problem with ECEF is that it would require two coordinate translations.  All your location data is in lat/lon/elevation.  You'd have to translate that to ECEF and then do the reverse after your computation.  That's all just added work.  As for the difference between geodesics and great circles, the difference may be a few miles for most routes but it does impact ATC when they expect everyone to following the same route.  And technically, the great circle would require a translation from/to WGS84.

Re the problem with using ECEF coordinates. The comment by Gerry is correct, but not complete. If you know the Lat/Lon coordinates of two airports that are far apart, then the great circle and ellipsoidal earth distances are readily calculated. You could convert both Lat/Lon pairs to ECEF, are you then going to use the Pythagorean theorem to find the distance between the airports? If you do, you're calculating the straight-line (Euclidean) distance between the airports.

In other words, total sum of the lengths of 3D line segments gives the length of fligth path (3D polyline).

The WGS84 standard defines the Earth as an ellipsoid with a semi-axis a = 6378137 m at the equator and a semi-axis b =  6356752.3 m at the pole.
The circumference of that ellipse can be approximated by:
C(app)= π (3(a + b) – ((3a+b)(a+3b))^0,5)
C(app) = 40007862.2 m
Now, if we take an sphere with r = (a+b)/2 = 6367444.65
C = 2πr = 40007834 m
The difference is 28.21 meters for the full circumference. Hence, for the usual distances in aviation, the difference will be of the order of a few meters, and that in the worst case…


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Why is the velocity squared in the lift equation?
Why is the velocity $v$ squared in the lift equation?
$$L=C_L\cdot\frac{1}{2}\cdot\rho\cdot v^2\cdot S$$

Can you share the formula you are asking about?

At a very high level of abstraction, it's because you're hitting more air per unit of time (proportional to v) and you're hitting each bit of air with more momentum (also proportional to v). (At least, that's why drag is proportional to v^2; I assume it holds for lift.)

Kinetic Energy is why the V2 is  there.  The formula for kinetic energy is:
$$E_{kin} = \frac{1}{2}\cdot m\cdot v^2$$
and the greek letter rho is the density of the air which is mass per unit volume.  So you can see the formula for kinetic energy in the lift equation.  Cool huh?  So what the lift equation is telling you is that the kinetic energy of the wing moving through the air is responsible for lift and we all know that you need velocity to lift a plane off the ground.  This also tells you that velocity is the biggest contributing factor to the lift component so airspeed gives you lift more than anything else.


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What orders of Ram Rise Coefficient (Recovery Factor) are commonly encountered?
The answer to the question about OAT, RAT, TAT, and SAT has the following:

Problem is, not all instruments will compress that air in the same way, and not all of them will pick up this Ram Rise entirely. So they will publish a K, a ram-rise-coefficient (also called recovery factor) meaning how much of from the theoretical Ram Rise they actually pick up.

It appears for speeds exceeding 1.5 Mach, Ram Rise starts achieving oppressive values. At 1.6 Mach, and -25C SAT we're already dealing with temperature rise to above 100C. The only "redeeming factor" is K, which may prevent such rapid and drastic rise for a short while.
What orders of magnitude does K appear in, in typical scenarios where ram rise starts playing a role - a supersonic fighter, a missile, a rocket?


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Does the crew have a professionally mandated responsibility to inform passengers of a potentially fatal emergency?
This follows on from the question here on an incident wherein a passenger asked crew about an aberrant situation during a flight.
Gathering from the answers to the question, flight crew's priority is to handle the emergency, and part of that is keeping passengers cooperative and calm, whether by informing them of details or perhaps omitting them in the interest of maintaining decorum.
However, in the case of a potentially fatal emergency, wherein passenger death is probable, despite the interest to maintain calm, does the crew have a professionally mandated responsibility to inform passengers? There are obviously moral and ethical motivations for this, but these might be beyond the scope of the site and as such I am asking how these issues are dealt with.

By "professionally mandated responsibility," it can be by laws in the appropriate jurisdiction, regulations and internal policies by the airline, common practice, or guidelines set by an aviation body.

Can you define what a "professionally mandated responsibility" is? Are you talking about regulations? Moral obligations? Just "what is expected from a professional"?

If the passengers are likely to die, then the crew is likely to die as well, and there would be no means left for the airline to discipline or sanction the crew for failing in some "professionally mandated responsibility"; so its a moot question.

@RonBeyer Either laws, regulations set by the airline or considered common practice by the personnel.

There are no laws (at least in the US) or regulations that say this must happen. The airline doesn't set laws or regulations but has SOP's which are not typically made public. That leaves "common practice" which may be hard to answer since those that tell the passengers "we are going to die, sorry", are usually not alive to testify that they did that.

The general professional practice is to try to keep control of the aircraft for as long as you physically can, i.e. until impact or you lose consciousness or whatever. Taking time away from that to tell the passengers that they're about to die - which you may not know with absolute certainty anyway - seems like it would be unprofessional, in the sense that you're reducing their chances of survival by not focusing on flying the aircraft. Whether or not you should tell someone they're about to die as a general moral obligation seems like it belongs on philosophy.SE rather than here.

I'm finding it really hard to come up with a scenario in which likelihood of death was high, yet there was time and opportunity to calmly tell the passengers about it, and the situation would not be made worse by having a whole load of panicking passengers.

@RonBeyer they may not bealive to testify but usually there is evidence of their last words.

Would the act of informing passengers they may die be considered "professional" or "a responsibility of the crew"? No.
And if I may argue, it's the opposite: it is unprofessional for trained flight crew (either pilot or cabin crew) to behave this way.
The shortfalls of this action is obvious. Panic would instantly spread among the cabin. Many people would start behaving irrationally when they believe that death is imminent (hey, not everybody is strong psychologically). This could potentially turn a survivable situation into a deadly one.
In dangerous scenarios, anyone assisting needs to give assertive instructions. We don't tell people why they are doing it. We give very simple, but firm instructions such as "push!", "jump!". Fire fighters and other rescue workers also do it this way.
Telling people "uhhhh, I don't think we can make it" works against the goal of ensuring everyone has a best chance of survival, which is a professional responsibility of any crew member on a passenger flight.

Legally, there are no regulations, industry practices, or airline SOP that I'm aware of, that mandates informing passengers about the potentially fatal outcome of their flight.
Realistically, there aren't that many scenarios where this is possible. If it's a small problem such as high oil temperature or failed hydraulic pump, you won't even notice. If it's a stall or loss of control you'd be screaming already. If it's a inflight breakup you'd loss consciousness in less than 30 seconds. If it's a cabin fire you'd see cabin crew fighting it with fire extinguishers.
That leaves scenarios where the aircraft is stable in the air, but may break up upon landing. Examples would include landing gear malfunctions, flap malfunctions (causing very high landing speed) and total engine failure from high altitude. Historically, most of these incidents end up well. Spending the time to educate passengers emergency procedures (such as counting the number of rows to various exits; briefing how to open the emergency door) is more constructive then saying "On behalf of XXX Airlines, it is with sincere regret to inform you that our aircraft suffered a catastrophic failure and this may be the last moments of your life".

That said, I offer the following:
If you've been told to adopt the brace position, you may consider that it is in the opinion of the flight crew that severe injury and/or death is likely.

Many people would start behaving irrationally when they believe that death is imminent https://youtu.be/Z9QTMTlxnq0

I know of exactly one airliner accident where the crew knew well in advance that fatalities were likely: Varig 254 (https://en.m.wikipedia.org/wiki/Varig_Flight_254). Due to several navigation errors, the airplane became lost over the Amazon rainforest. The crew announced what was going on shortly before the crashed landing. There were 13 fatalities (all passengers) out of 54 occupants.

If there is a catastrophiv failure duch as inflight break up of a wing or a structure and communication is possible..I would say "Brace and lets pray". Not saying anything and knowing death is inevitabpe is also cheating them of their last moments to say or do what they desire..if only to think of their loved ones!

This is very probably the moral thing to do, but the question asks about professional mandate.


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External recordable CCTV, a third black box
I wonder if aircraft, and especially large commercial ones, should be fitted with external recordable CCTV. This would enable the crew to perform visual pre-flight checks from the cabin that would not normally be possible to ensure that flaps, rudder etc are actually working and to full extent, but also in case of emergency, would enable the crew to visually check the entire aircraft for the same, and for engine fire, for trauma etc at a glance. Valuable time can be wasted trying to figure out what has actually happened. This would also help prevent errors such as shutting down the wrong engine in case of fire. The recordable aspect would also act as an extra "Black box" in case of tragedy  and would no doubt be a great aid to and speed up Crash Investigation.
This system has been employed for many years in the bus and lorry industry, but mainly to help determine blame in case of accident.

Yes indeed. Safety should always be the number one priority, anything to reinforce safety cannot be dismissed. Perhaps publicity and public opinion might help.

@Paul It's easy to throw phrases like 'number one priority' around but there's always a compromise. The safest thing to do would be to simply not fly at all - that's the only way to near guarantee not being involved in an air accident. But, you compromise because the risk:reward is worth it.


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Where are the black triangle'd seats?
There are two windows marked with a black triangle, which indicate that they provide the best view for the wing, when the crew needs visual.
Since A320s are probably the most popular birds out there nowadays, in which row are these best-view windows?
It would be nice to take that seat when I check-in in my next flight.

What does this black triangle placard on board an A320 denote?

Row 9 then @mins, thanks and sorry for failing to find the dupe earlier.

Doesn't that depend on the seat configuration ordered by the company buying the aircraft?

@αCVn probably you are right, but it should be somewhere nearby... ;)

I wrote a real nice answer to this, went to click post, and found out you closed it..

@Matt I am really sorry, but I didn't have a clue. I now see that I cannot reopen the question by myself, it needs votes, etc...

Whelp, here it is in a pastebin - Note that it shows the triangle seat is not the best seat for passenger viewing.
https://pastebin.com/v7bG0Uic - I'll wait for the day the question "Is the triangle-marked seat best for passenger view?" pops up! :)

Not a dupe but maybe off topic.

@vasin1987 Not a duplicate, though the answer can be gleaned from the linked topic. A question such as "Which seat has the best view on an A320" is most certainly not offtopic.


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What are these small propellers in the A330 MRTT's wings?
In actual flight there are small propellers rotating at the front of these "pillars". What are they? They are visible in this video.

Those two locations you circled out are the under-wing refuelling pods, mounted on pylons (where, I suspect, the A340 outboard engines would be).
You can find a picture of them on the A330 MRTT Wikipedia entry, under the Design section.
And those propellers on the pods are likely the ram air turbines that power the refuelling pods, according to the manufacturer:

Wing-mounted aerial refuelling pods feature a drogue refuelling system and an integral fuel boost pump which provides a fuel transfer capability up to 450USGPM (1703 Litres per minute) at 45 to 55 PSI (310 to 379 kPa). Pods operate with aircraft power or are self-powered by a Ram Air Turbine.

You are correct about the location of the A330 refueling pods indeed -- the A330 MRTT uses the A340 wing for precisely that reason.

This is a very smart example of component-reuse. Put refueling pod onto the twin-engine A-330? Use the quad-engine A-340 wing. I guess this is an unexpected, positive consequence of the common development of the A-330 and A-340.

RAT


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Are passengers allowed to travel seatless(on the floor)?
I was told that a flight in Europe carried two passengers on the floor, because their purchased seats didn't exist. BBC.
Is that allowed?
For example, in Greece, a bus is not allowed to travel with any passenger on the floor (or equivalently standing), so allowing a flight to do so seems surprising!

You will see that sort of thing on 2nd and 3rd world airlines, but never in one operating to normal western standards.

This may vary by jurisdiction but its generally not allowed.

I am asking specifically for Europe.

I think you are referring to this incident. The passengers were actually sitting in crew seats during takeoff and landing and only had to sit on the floor while the crew needed the space to prepare the service. The CAA is investigating the incident.

Is those unseated passengers didn't make unstable the CG?

Passengers on commercial airliners (US, EU, Canada, most of the world) aren't allowed to sit on the floor because 1) they could block the aisles and emergency exits and 2) they cannot be buckled up, putting their and others lives at risk.
That does not mean it could not have happened, just if it did it was against regulation.
There are cases where commercial passengers may sit on the floor but they are rare, for instance parachuting.
EDIT: In the BBC article cited after the original post says that the passengers were given spare crew flip-up seats for take-off and landing, but had to sit on the floor the rest of the time so the crew could do their service. This is against regs as they would not have been secured against unexpected turbulence. The article says the following which proves my point:

The Civil Aviation Authority says while passengers are allowed to sit
  in crew seats under certain conditions, they must not be left unseated
  during any stage of the flight.

I didn't think of parachuting, but of course this is a special case. So, it's not allowed, thus the airliner should be punished.

That sort of thing makes the news believe it or not, I would want to have some evidence it happened. In other parts of the world it happens all the time, so maybe the story you heard was one of those cases.

Note that the question was later edited to include a link to a BBC story about the incident.  The flight was from Minorca to Birmingham, and the "missing" seats were due to an equipment change.  It sounds like the passengers were seated for take-off and landing, but that they couldn't/didn't keep their seats during the cruise phase.

Oh god, TUI! Don't get me started on those jokers, no seats and all they were offered was £30 in compensation!

@MichaelSeifert indeed. However, I don't get which seats are. I mean, on my flights, flight attendants are seating, and I don't recall any spare seats...
"The family were offered £30 (AU$53) by TUI as a ‘goodwill gesture’ and they offered to refund their tickets." src. However, passengers are usually shortsighted and almost always agnostic of the risks, they shouldn't be allowed to fly.

I can't believe the CAA won't prosecute them for this, it's an egregious example for TUI to set.  I imagine the CAA will want to make an example of them.  Imagine what will happen when Ryanair hear of this!


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First airplane to use a flight simulator?
Icarus, Pegasus, da Vinci's flying machine and Wright brothers airplane probably didn't have a flight simulator, since computer science was not yet developed enough (Antikythera ancient analogue computer in 70BC, Wright brothes until 1948, ENIAC introduced in 1946).
So, which bird was the lucky one, to first be accompanied by a flight simulator?

if you count the WW2 era Link Trainers, probably some early 1940s training aircraft like the Harvard.

@jwenting I didn't know about Link Trainers. At a glance, I cannot tell the difference between a fl. simulator. Every airplane is a candidate.. ;)

Planes do not use flight simulators. Pilots do. Any flight simulator has a limited accuracy. First flight simulators were not specific to a type. I have the feeling that the questions you want to ask is: What is the first airplane that has been modeled in a flight simulator with relative accuracy?

Hmm @bogl not really. I am interesting for the airplane that was flown by pilots that were trained in a fl. for the first time. Mentioning whether that simulator was generic or not, would be of course nice.

So your asking what plane was the first to have a dedicated flight sim that trained pilots?

@Alex nicely put. Maybe the title of my question should be your comment...

According to the Oakland Aviation Museum the first known was a simulator to learn to fly the Antoinette monoplane, although they don't provide any references for this, so I can't say that is entirely true.
The link trainer is the best known trainer and they were built in 1927 by Edwin Albert Link. They were used to train Army Air Corp pilots for instrument training in all weather conditions. The link trainer continued to be the sought after training device in WWII as well training almost all US Air Force Pilots.Link
If I find more information I will update my answer as best as possible.
I know typically answers on Wikipedia aren't the best but this one is littered with references that would take entirely to long to search out for ones self.

I can't find a source but I'm pretty sure that the first modern airliner type sim was developed for the DC-4/6.   There is a clip somewhere of Arthur Godfrey's show in the mid 50s he did from the terminal at Idlewild Airport (JFK) that included a visit to a training sim, which was a DC-4 or 6 IIRC.  Maybe fish around in that direction.

I think this is the source you are looking for


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What is that letter in the top of the tail of that Jumbo?
Aristotle Socrates Onasis, after marrying Ja. Kennedy, bought, from Boeing, a 742, the Olympic Eagle - now left in Hellikon Airport to rot, after the Olympic Airlines went public ..
At the top of its tail the character 'B' appears, what does it mean?

What’s the aircraft‘s registration?

I don't know what a registration is, but maybe SX-OAB? Could that last character be that B?

It could well be the last letter of the registration. Wikipedia has a picture of SX-BKH and it has an "H" at the top of the tail.

And here is SX-DFC an A340 with a "C" on the tail. Not conclusive proof, but a very likely candidate. Unfortunately, this scheme limits them to a max of 26 aircraft at a time, and they need some cooperation from the CAA to get the registrations right. :)

@FreeMan same for the B727 next to the Jumbo, 'A' is at the top of the tail, and the last character of the registration's string, see video.

@FreeMan Since you could probably identify the aircraft model by the tail, I‘d observe that that scheme limits the available registrations per aircraft model to 26, so it might have sufficed for that particular airline...

I noticed this Iranian ATA Airlines 737 had the last letter right on the nose! https://www.jetphotos.com/photo/9078184

Hmm @zymhan, maybe the most visible part is the nose in their setting, nice observation!

To identify the aircraft from a distance.. eg if you are driving on the ramp and need to get to SX-OAB and there are several Olympic jumbos parked up, and from where you are you can't read the registration on the fuselage. Not too bad if they're in the same general direction but if some are on your right and some are on your left it may help if you could get a hint from the tail. They only had one letter as OA never had that many widebodies.. I think they never had more than 3 747s at the same time and some A300s. Lufthansa on the other had uses the last two letters. Probably because they had 400s (pax and combis) and 200s at the same time. The pax 400s started at D-ABVA, the 400 combis started D-ABTA and the 200s were D-ABZ(x)(Freighters) or D-ABY(x)(pax).
More airlines put the registration on the nosewheel doors, this saves the ramp staff from trying to get a glimpse of the tail (not easy in bad weather). It is also common to see the last one or two digits/alphabets on the front end, most commonly on the nosewheel doors but also above the cockpit windows or just behind the radome.